Variable Speed System Pumping Theory & application “Why would I use one of these things?

Why Use a Variable Speed Circulator? Good question… What do they do? Vary speed based on changing loads Use external information Best applications… System circulator with zone valves Radiant with multiple zones on single manifold using actuators Why would you use a variable speed circulator in a modern hydronic heating system? And what would the benefit be to the contractor or the ultimate end user? Both are excellent questions, and the answers aren’t as obvious as you might think, but in the right system, the benefits can be tremendous. First, we needed to understand what variable speed circulators do. As the name might suggest, variable speed circulators vary their speed – going faster or slower as the heating loads of a structure change during the course of a day. A Variable speed circulator uses external information to determine how fast it should go – and that information, ideally, should reflect the specific heat loss at a given point in time. Variable speed circulators don’t fit into every job. The best applications include zone valve systems with one system circulator, and radiant floor heating systems using manifold actuators –with multiple zones on a single manifold.

Universal Hydronics Formula GPM = BTUH ¸ DT x 500 GPM = Gallons per minute BTUH = Heating load DT = Design temperature drop 200 for baseboard 100 for radiant 500 = 8.33 x 60 x 1 What’s the point behind variable speed circulators? First understand that the heating load literally changes with the weather. As the outdoor temperature changes, the overall heating load of the structure changes. In addition, when you have multiple zones in a heating system, the load at any given point in time will change based on how many zones happen to be calling at that moment. To fully understand this concept, we need to understand the Universal Hydronics formula, which states that GPM equals BTUH divided by Delta T times 500. Let’s define the terms. GPM is gallons per minute – it’s the flow rate required to deliver a specific amount of heat. BTUH is the BTU per hour requirement at a given point in time – the heating load. Delta T is the designed temperature drop across the piping circuit. In a series loop fin tube baseboard zone, the design Delta T is 20 – that’s the manufacturer’s rating for the baseboard itself –meaning the water might enter the baseboard zone at 180 degrees, and return to the boiler 20 degrees cooler, at 160 degrees. In most residential radiant floor heating systems, however, the design Delta T is usually 10 degrees – meaning the water enters the radiant loop at 130 degrees and returns at 120 degrees. This 10 degree Delta T is important in radiant because it ensures an even, comfortable floor surface temperature throughout a room. A wider Delta T might mean a noticeable drop in floor surface temperature near the end of the run. The final element of the equation is 500 – that’s a shortcut representing the weight of one gallon of 100 percent water – 8-point-33 pounds – times 60 minutes in an hour --- times the specific heat characteristic of the fluid – which is 1 for 100 percent water -- it takes 1 BTU to raise the temperature of 1 pound of water 1 degree in 1 hour. 8-point-33 times 60 times 1 is 499-point-8 – we’ll call it 500. Remember that virtually everything in hydronics – from pipe sizing to circulator selection – stems from this formula.

Sample Project Heat loss = 75,000 BTUH Design temp = 00 3 zones fin tube 25K BTUH each 200 DT Let’s apply the universal hydronics formula to a sample project. This one will be very simple – a basic residence with an overall heating load of 75,000 BTU’s at an outdoor design temperature – the coldest day of the year – of 0 degrees. The job will include three zones of fin tube baseboard – and each zone is of equal size – 25,000 BTU’s each. And since we’re using fin tube baseboard, we’ll calculate the flow for they system based on a 20 degree Delta T.

Do The Math! GPM = BTUH ¸ DT x 500 GPM = 75,000 20 x 500 GPM = 7.5 10,000 Let’s plug the numbers into our formula: Remember that GPM equals BTUH divided by Delta T times 500. In this case, GPM equals 75 thousand divided by 20 times 500. 20 times 500 is 10 thousand 75 thousand divided by 10 thousand gives us a flow rate for the job of 7-point-5 gallons per minute.

For Each Zone… 25,000 BTUH each GPM = 25,000 20 x 500 GPM = 2.5 per zone 20 x 500 10,000 Let’s do the same for each zone: 25,000 divided by 20 times 500 – or 10 thousand – equals a flow rate per zone of 2.5 gallons per minute.

Pipe Sizing 2-4 GPM = ¾” pipe 4-8/9 GPM = 1” pipe Min/max velocity 2-4 FPS > 4 = velocity noise Armed with this information, we can now size some pipe. Using these pipe sizing guidelines, we can determine the proper pipe size for the boiler supply pipe and the boiler return pipe, as well as the distribution headers. These guidelines are based on minimum and maximum flow velocities – a minimum of two feet per second, and a maximum of four feet per second. If we exceed four feet per second, the likely result will be velocity noise. Keep that in mind later on.

Let’s Pipe ‘Er Up! ¾” 1” Here’s a look at the piping arrangement for the system… Using our guidelines, we’ll see that we’ll need one inch pipe for the boiler supply and return and for part of the header. At the header, we can branch off into three three-quarter inch lines for the baseboard zones. We’ll need to do the same thing only backwards on the return side of the system.

Head Loss 150’, including element Next, we need to estimate the head loss of the piping system so we can select the proper circulator. To do this, we’ll need to measure the longest zone from the outlet side of the circulator all the way around to the suction side of the circulator. For our purposes, let’s presume the longest run is 150 feet of pipe, including the baseboard element.

Head Loss Longest run = 150’, including element Multiply by 1.5 to allow for fittings, etc 150 x 1.5 = 225’ Multiply by .04 4’ head/100’ of pipe 225 x .04 = 9’ head loss 7.5 GPM @ 9’ head To estimate the head loss, take the length of the longest run – 150 feet – and multiply it by 1-point-5 to allow for additional pressure drop through fittings, valves and other stuff that’s in the way. If we multiply 150 times 1.5, we come up with a total Equivalent length of 225 feet. Next, we multiply that number by point-zero-four. That number represents 4 feet of head loss per 100 feet of straight, properly sized pipe. That’s based on the maximum flow velocity of four feet per second. 225 times point-zero-four equals 9 feet of head loss So our circulator will need to be sized to provide 7-point-5 gallons per minute while overcoming 9 feet of head loss. Note – we need to size the circulator for the total flow rate required for the job – 7-point-5 gallons per minute – because it needs to deliver 75 thousand BTU’s. But with head loss, we only need to size to the worst head loss zone. We have, at this point, a parallel piping system – so if the circulator can overcome the head loss in the worst case zone, it can certainly overcome the head loss in all the others. We don’t need to add all the heads together

Pick The Pump… 15 Taco 008 Pump Curve 10 5 20 9’ Head 7.5 GPM So let’s pick the circulator. We’ll need to provide 7-point-5 gallons per minute at 9 feet of head Taking look at this pump curve chart – we find that a Taco 008 would be a solid choice – being able to deliver 7-point-5 gallons per minute at 9 feet of head loss. 7.5 GPM

System Operating Point – System Curve 5 10 15 20 System Operating Point – All Zones Calling Now it’s time to understand something called a system curve. We know two points on the curve, already. At 7-point-5 GPM we have 9 feet of head. And at zero GPM we have zero feet of head. We can calculate other head loss points at other flow rates, and plot them on the pump curve graph. And we can see that the actual operating point of the system will be where the system curve intersects the pump curve – at this point here. So far, so good But let’s think inside the system for a moment. The system requires 7- point-5 gallons per minute only when all zones are calling and only when it’s ZERO degrees outside…

As Zone Valves Close… 15 15 10 10 5 5 20 20 System Operating Point – 2 Zones Calling System Operating Point – 1 Zone Calling 5 10 15 20 5 10 15 20 But what happens when zone valves close? Well, the structure obviously requires fewer BTU’s. Say only 2 zones are calling – the BTU requirement drops to 50 thousand. If only one zone calls, the requirement drops to only 25 thousand. As zone valves close, the system still has to operate on pump curve, as shown. New system curves are created. You’ll note where these new system curves intersect the pump curve – we will actually have higher flow rates running through the system than would be needed. So what happens? 6.5 GPM 4 GPM

System Operating Point – As Zone Valves Close… System Operating Point – 2 Zones Calling GPM = BTUH 6.5 = 50,000 DT is lower than design Poor heat transfer Less efficient boiler Velocity noise 5 10 15 20 DT x 500 DT DT x 500 Let’s return to the Universal Hydronics Formula. We know that GPM equals BTUH divided by Delta T times 500. Looking at our charts, we see that the GPM provided by the circulator with only 2 zones calling is fixed at 6-point-5. The BTU’s needed with 2 zones calling is only 50 thousand. The 500 in the equation is fixed – so what’s the only thing that can change? That’s right – the Delta T. It will, in fact, not be 20, but something considerably less. With only one zone calling, the Delta T will be lower still. As the Delta T narrows up, and the water temperatures going back to the boiler are higher than system design – it’s certainly possible, if not likely, for the boiler to short-cycle, even in the dead of winter. And that short-cycling, of course, gravely affects overall efficiency for a cast-iron boiler. And imagine sending higher water temperatures back to a modulating-condensing boiler than you intend. That will certainly affect its overall efficiency. Also of concern is the amount of pressure differential being created in the system. As you can see, as zone close, the system curve intersects the pump curve at higher and higher pressure differentials. This greater pressure differential can cause greater flow velocities within the system that will most likely lead to excessive -- and unwelcome – velocity noise. So, with a fixed speed circulator, it’s certainly conceivable to have a system that has relatively poor heat transfer, isn’t as efficient as it could or should be, and be objectionably noisy. 6.5 GPM

So What Do We Do About It? Ignore it? “That’s just the way these systems are!” So what do we do about it? Traditionally, the answer has been to either ignore the problem, or just live with it – especially the noise. For the most part, no one would notice relatively poor heat transfer and wouldn’t necessarily notice a slight loss in system efficiency. If the customer was replacing an old boiler, they would be enjoying lower fuel bills anyway, and wouldn’t notice that those savings could have been even better. But with today’s energy situation and with today’s hi-efficiency heating equipment, it behooves us to make sure of the water temperature we’re sending back to the boiler. Our customers are spending major dollars on the hi-efficiency equipment, but we may be short changing them on the actual savings

A Good Solution… There are some solutions. One way to deal with the noise issue would be to install a pressure differential bypass valve. Here’s an example of one added to our system. It’s a spring loaded valve that is adjusted to prevent flow through the valve when all zones are calling. But as zone valves close – and the pressure differential goes up – the bypass valves opens and allows that excess pressure – and flow – somewhere to go. As more valves close – the valve opens even more.

A “Flat” Solution The bypass valve is a good solution – for noise. A better solution for noise would be a flat curve circulator – like a Taco 007. As you can see – as zone valves close the system pressure does not increase much at all – preventing the build-up of pressure differential and eliminating the need for a bypass valve. The 007 is a high flow, low head, flat curve circulator. If a job has higher head requirements – such as the job we’re looking at – we’ll need to look elsewhere for our solution.  

System Operating Point – A Better Solution… Variable speed pumping! 5 10 15 20 System Operating Point – All Zones Calling That leads us to another option – and a better one – a variable speed circulator. You’ll note back when we originally size dour circulator – a Taco 008 – the system curve representing all zone calling intersected the pump curve here – and that the actual flow rate wasn’t 7.5 GPM – but really about 9 GPM. Actual flow rate » 9 GPM

Universal Hydronic Formula GPM = BTUH BTUH = GPM x DT x 500 DT = BTUH DT = 75,000 DT = 160 DT w/2 zones = 150 DT w/1 zone = 120 Under DESIGN conditions! DT x 500 GPM ¸ 500 Let’s look again at the Universal Hydronics Formula – GPM equals BTUH divided by Delta T times 500. A little algebraic manipulation shows that BTUH equals GPM times Delta T times 500. A little more algebraic manipulation tells us that Delta T equals BTUH divided by GPM, divided by 500. Let’s plug in the numbers we know. With all zones calling, GPM equals 75 thousand divided by 9, divided by 500. If we math that out, we find that the actual system Delta T at this point would be 16 – not the 20 we designed for. Is this a big deal? Well, that’s a difference of 20 percent ! With only 2 zones calling, the actual Delta T would be about 15 degrees – a 25 percent difference. And with only 1 zone calling – the actual system Delta T would only be about 12 degrees – a 40 percent difference. And those, remember, are the numbers when it’s ZERO degrees outside. What if it’s – say – 35 degrees outside – and the heat load at that temperature is only 37, 500 BTU’s with all zones calling? You can see the potential for smaller and smaller Delta T’s, over 60 percent differences to design, greater boiler short-cycling and increasing velocity noise in this simple system. 9 ¸ 500

But What If DT Was Fixed? GPM = BTUH GPM = 75,000 DT x 500 GPM = 7.5 The flow will vary! DT x 500 = 50,000 = 25,000 20 DT x 500 2.5 5 But what if the Delta T was fixed? Again, using the Universal Hydronics formula –we’ll find some interesting truths. If we FIX the Delta T as 20 – and divided the total load of 75 thousand by 20 times 500, or 10 thousand – we find that the flow rate has to be – HAS TO BE – 7-point-5 gallons per minute. With 2 zones calling – a load of 50 thousand BTU’s – we find that the flow rate HAS TO BE 5 gallons per minute. And with 1 zone calling – the flow rate HAS TO BE 2-point-5 gallons per minute. Obviously, with a fixed Delta T – the flow will have to vary as the zones satisfy. In addition, in spring, fall, or at any time when the heat load is less than maximum, the flow will have to vary to maintain that fixed Delta T

A Better Solution… 5 10 15 20 So, instead of using a fixed speed circulator, use a variable speed circulator – one that varies its speed based on the simple, understandable and essential element of hydronics – Delta T – or system temperature difference. If the Delta T is fixed – and the BTU requirement drops – the GPM has to drop with it! Also, under design conditions, you’ll never have to worry about over-sizing the circulator because a Delta T circulator will always maintain the Delta T. So instead of working at the point where the system curve intersects the pump curve – as shown here… …the pump will actually slow down and SELF-ADJUST to the proper operating point --- every day of the heating season…. …as zone valves close, or as it gets warmer outside and the heating load goes down…the circulator will vary its speed accordingly. The Delta T circulator does, in fact, have an infinite number of pump curves.

An Important Consideration… Calculate max flow rate Estimate head loss Length X 1.5 X .04 Head loss at MAX flow rate Actual flow rate much less Can overestimate head by 50% DT circulator “self-adjusts” When choosing a variable speed circulator – there is an important element to consider. When we sized our circulator, we based it on the maximum flow rate – which was based on the heat loss. Next, we estimated the head loss – key word here is estimated. It was based on the length of the longest run, multiplied by 1.5 to allow for fittings – again an estimate – and then multiplied by .04 – for 4 feet of head per 100 feet of pipe. This formula will estimate the head of the MAXIMUM flow rate and MAXIMUM velocity for any given size of pipe. In our examples, however, the calculated flow rates were much less than the maximums. The maximum flow rate for the three-quarter inch zones was 4 gallons per minute. The calculated flow rates under design conditions were only 2-point-5 GPM per zone. The flow rate requirements would be even less when conditions are milder than design. And that’s assuming we didn’t over-estimate our heat loss in the first place. This simple rule of thumb head loss formula can actually over-estimate the head loss in the system by as much as 50 percent. This could be an issue with a circulator that varies its speed based on Delta P – or difference in pressure – since it needs to be programmed based on maximum system head loss. A Delta T circulator, however, works based on what’s really going on within the system at any given point in time, and will self-adjust to always provide the proper flow rate.

Taco 00-VDT Variable speed DT circulator Control built in Simple to install Simple to program Simple to understand! 008, 0012, 0013 VDT So let’s take a look at the Taco 00-VDT – a variable speed Delta T circulator. The Delta T control is built right in – and it’s very simple to install, incredibly easy to program, and based on what we’ve learned so far, easy to understand, as well. The 00-VDT is available in three standard models – the 008 VDT, the 0012 VDT and the 0013VDT – these three models should handle virtually any residential hydronic system you’re likely to encounter – including baseboard and radiant floor heating.

A Simple Solution 00-VDT For installation – there’s nothing new or different – install it as shown here in a simple system. The only extra work is wiring the sensors to the supply and return, as shown

Setting It Up… Dip Switches Range Dial Switch 1, 3 ON All others OFF Adjust to desired DT It’s a very simple circulator to set up, as well. There’s no magic involved, and the instructions are written in plain English. To set up the 00-VDT – you’ll first look at the Dip Switch panel, shown here… Set Dip Switches 1 and 3 to the ON position – all other Dip switches should be left in the OFF position. But then again it comes from the factory pre-set that way. Next, notice the range dial in the upper right hand corner of the control panel…. …with the Dip Switches properly set – this range dial can be read in 5 degree increments…meaning each notch on the dial represents a 5 degree differential. So if you’re designing a radiant floor heating system at a 10 degree Delta T, simply set the range dial to 2. For a 20 degree Delta T, set the range dial to 4…and so on.

Wiring It Up… H-N to ZVC Supply sensor to S2 & Com Return sensor to S1 & Com Supply Sensor To power this bad boy – simply wire the H and N terminals to power – either to a single or multiple pump relay or to the end switch on a Zone Valve controller… Then, wire the supply sensor to terminals S2 and COM….and the return sensor to terminal S1 and COM. You’ll also note a jumper connected to the HT REQ terminals. This jumps the internal heat demand – and allows the circulator to be turned on and off through a relay or zone valve controller by simply breaking the line voltage. If the circulator is to be powered constantly, remove the jumper and connect the HT REQ terminals to an end switch. Supply Sensor Return Sensor Return Sensor

Delta T vs. Delta P ΔT directly related to flow rate Pump speed adjusts to required BTU/hr GPM = BTUH/ ΔT x 500 ΔT always runs at lowest required speed Delta P adjusts only to pressure change Needs accurate head loss calculation – not estimation Doesn’t know which zones are open/closed May not be able to satisfy heat demand Finally, we’d like to point out some differences between the Taco 00-VDT circulator, which varies its speed based on heating system Delta T, and other variable speed circulators, which vary their speed based on Delta P, or difference in system pressure. First off, Delta T is, of course, directly related to the required flow rate in a heating system. The amount of flow required at any given point in time is based upon the BTUH heating load at that given point in time, divided by the system Delta T times 500. By maintaining a fixed Delta T within the system – which is achieved with a 00-VDT circulator – the flow rate produced by the circulator will vary with the heating load of the structure at any given point in time. If it gets colder outside, and the BTUH load of the structure goes up, the circulator will go faster because it needs to deliver more BTU’s…and increasing the flow is the way to accomplish that. As it gets warmer, the circulator will slow down, because the heating load goes down as it gets warmer…and the flow rate delivered will decrease. Also, the circulator will speed up or slow down as zone valves open or close – when a zone is satisfied, the structure doesn’t need as many BTU’s – the 00-VDT will slow down at that point to reduce the amount of flow, and resulting BTU’s, being delivered. In fact, the 00-VDT will ALWAYS work at the lowest speed required to deliver only the BTU’s required at a given point in time. A variable speed circulator that varies its speed based on Delta P, or difference in pressure within a system, will speed up or slow down when zone valves open or close, but only to maintain a programmed pressure differential in the system. Accurate headloss needs to be calculated – not estimated – for the Delta P to be accurate. In addition, a Delta P circulator doesn’t know which zones, or combination of zones, are open at any given point in time. For instance, if a radiant system has four zones, all equal loop lengths, but the rooms have different heating loads, the circulator will only care that two of the zones are calling, and two aren’t. If the two high heat loss zones are calling, the circulator won’t be able to provide enough flow to satisfy the demand.

Energy Consumption 137 kW kW consumed, 117 kW 2,500 hour heating season Assuming 12.5 hours of operation daily 117 kW 36 102 kW 33 ECM circulators draw constant 5 kW in “standby” mode to keep memory powered – for 6,260 non-heating hours annually We would also like to discuss the issue of energy consumption, and the different types of motors available in variable speed circulators. The Taco 00-VDT circulator has a split capacitor AC induction motor, which uses anywhere from 25 to 90 watts while running, depending on its speed. Delta P circulators have ECM – or electronically commutated motors, which use anywhere from 5 watts to 59 watts, depending on speed. While an ECM motor does use less electricity, when you’re dealing with partial horsepower motors – and here we’re dealing with 1/25th horsepower motors here -- the real difference in dollars and cents is minimal. This chart shows kilowatt hours consumed by the three leading variable speed circulators – the first two are Delta T circulators using ECM motors, the third is the Taco circulator using a split capacitor AC induction motor. As you can see, the ECM motors use up to 50% less electricity. However, the electronics on these circulators require a constant 5 watt draw so that they may detect a difference in pressure when there’s first a call for heat, and that draw is 24-7 during all “non-operating” hours, all year round. The Taco 00-VDT, however, only draws power when there’s a call for heat. When there’s no call for heat, it doesn’t draw any power. And it can be controlled by a standard pump relay or zone valve controller. When all the variables are factored in, you can see there is still a difference in consumption, but it’s considerably smaller…between only 15 and 25%. Taco 00 VDT draws NO power when off – runs off standard relay or zone valve controller

Cold, Hard Facts… 25% Savings!! WOW!!!! Dude, that’s only $3.16 per year… So what does that mean in terms of actual dollars and cents? Here are the cold, hard facts… Based on 2500 hours of operation – and 6,260 hours of standby – at the national average of $0.09per kWhr, the ECM circulators cost between $9.18 and $10.53 PER YEAR to operate, compared to $12.32 PER YEAR to operate the Taco 008-VDT. The supposedly more efficient ECM motor saves roughly $3.16 per year over a conventional motor. Sure, that’s a savings of 25%, but the fact remains it’s $3.16. For the year. To further put this into perspective, you would save more than that if you changed just one 60 watt incandescent bulb to a compact fluorescent bulb – and this is pretty conservative. If you changed 10 conventional bulbs to compact fluorescents you’re way ahead of the game.

What About System Efficiency? Big difference between Delta T & Delta P Cast iron boilers Delta T circulator controls return water temperature Reduces short cycling Uses less fuel Bigger impact with “mod-con” There is, however, major difference between Delta T and Delta P when it comes to how efficiently the boiler will operate. A 00-VDT Delta T circulator will always send the lowest possible water temperature back to the boiler, since it will slow down to maintain the designed-for Delta T. For example, with a cast iron boiler and a baseboard system designed to a 20 degree delta T, the water temperature going back to the boiler will be 20 degrees cooler than when it left. That means the boiler won’t short cycle nearly to the degree than it would with a Delta P circulator, which does nothing to control the system Delta T. And when a cast-iron boiler short cycles, its burns less efficiently, using more fuel than necessary. With a modulating-condensing boiler, the differences are even more dramatic.

Delta T & Boiler Efficiency With a modulating-condensing boiler, the key to maximizing efficiency, and reducing overall fuel usage as much as possible, is to control the boiler’s return water temperature. Here’s a chart showing boiler efficiency for a hi-tech mod-con boiler. Notice that you get the most efficiency at the lowest firing rate combined with the lowest return water temp. Also notice that a few degrees change in the return water temp can make a significant difference in the overall thermal efficiency. Check out the lowest firing rate. If the return water temperature is 130 degrees instead of 135 degrees, the efficiency increases from 88% to 94%. At 40% input the efficiency goes from 88% to 89%. Return water temp is critical to achieving maximum boiler efficiency and maximum boiler efficiency is the key to maximum system efficiency. A Delta P pump WILL NOT ensure that you get the correct temperature drop across your system. In fact, with only a few small zones open the chances are that your flow rate will be greater than what is ideal and therefore your return temp will be higher than what is ideal. The only way you can ensure that your return temp will be where you want it – where you get the most efficiency – is to use a pump that varies the speed based on the DeltaT – not the Delta P. So, how much effect does a few degrees difference in return temp and a few points difference in efficiency have on your annual operating costs?

A Quick Comparison… Here’s a bar chart showing annual operating costs for our 75,000 BTU boiler with a variable speed pump. System 1 is operating at 93% efficiency and System 2 is operating at 90% efficiency. 90% is nothing to sneeze at and the difference doesn’t look that big in chart, but it’s actually about $37 per year in higher natural gas costs. Keep in mind that natural gas is the least expensive fuel you can use for home heating. If you’re using fuel oil or propane the savings get more dramatic. Even if you subtract the $3 difference in operating costs between an ECM motor and standard motor, in 5 years you still save $165 by using a DeltaT pump on your heating system, compared to an ECM Delta P pump. This isn’t counting any inflation for fuel costs and it’s not very likely that fuel costs will stay the same for 5 years. Keep in mind that is only for home heating. If you use an indirect water heater with your boiler – definitely a good idea – your water heating costs can approach the cost of home heating. Adding a DeltaT pump to the water heater coil circuit can help your boiler operate more efficiently during that cycle, too.

Some Parting Thoughts… ΔT directly related to flow rate Pump speed adjusts to required BTU/hr ΔT ensures optimum performance of BB, radiant, etc ΔT does not “flat line” ΔT always runs at lowest required speed Lots of models EASY to set up! Finally, some parting thoughts on variable speed pumping. Understand that Delta T is directly related to flow rate. It’s part of the universal hydronics formula – GPM equals BTUH divided by Delta T times 500. Delta P – or pressure differential – isn’t part of the equation. What we’re trying to do is satisfy the heat loss of the structure in the most efficient way – and the best way to do that is allow the circulator to adjust its speed to deliver the required BTU’s. By maintaining a consistent Delta T – 10 for radiant – 20 for baseboard – or higher for panel radiator systems – we can vary the flow as needed to ensure optimum performance and heat transfer… Another thing about Delta T – it doesn’t “flat line.” A delta P circulator will maintain a constant Delta P in the system – regardless of what the system actually requires…and if the Delta P that’s programmed into the circulator is not accurate – the actual flow rates in the system may be higher than required – which means a smaller Delta T than design. A Delta T circulator, on the other hand, will always run at the lowest speed possible for any given situation – maximizing system performance – especially efficiency back at the boiler. In addition, you have three standard Delta T circulator to choose from, depending on the size of your system. And most importantly, as you can see – the 00-VDT is very easy to set up…and you’re programming it based on a real element of heat transfer and system design – Delta T